1. Field of the Invention
The present invention relates to a rolling bearing unit having a metallic pulley for slidably supporting a belt on an outer surface thereof, and a rolling bearing for rotatably supporting the pulley. More particularly, the rolling bearing unit of the invention preferably has a metallic pulley with an outer surface which has a belt power transmission surface formed thereon.
2. Description of the Related Art
Driven pulleys are mounted on the engine auxiliary equipment of a motor vehicle, such as an alternator, an electromagnetic clutch and a compressor. A rotation of a crankshaft is transmitted to the auxiliary equipment by drive pulleys and belts. Generally, the driven pulley is formed of an iron, which has a high strength and is easy to be machined, such as S45C, a cold rolling steel plate, or SPCC. Recently, a resin pulley made of glass-fiber reinforced phenolic resin is also used for the driven pulley for the purpose of weight reduction. In the light of pulley form, the V-ribbed pulley with V-grooves formed in the outer surface is mainly used. The pulley made of a metallic material is usually coated with an anti-corrosive or wear-resisting paint.
Another pulley is proposed and disclosed in Japanese Patent Unexamined Publication No. Hei 3-189340. In the proposed pulley, a surface of a portion at which the pulley is rotatably mounted is subjected to hard plating coating, and a V-groove surface of the pulley is subjected to soft electrodeposition coating. Therefore, it is possible to prevent formation of fletching caused when the paint comes off by a friction between the belt and the pulley, and generation of a squeal by the belt.
However, where the general iron pulley is used near the engine as described above, a static electricity of about 10 kV is generated by a friction between the belt and the pulley, which are driven by the crankshaft rotating at higher speed. Part of the static electricity generated discharges to the air. Another part of the static electricity also discharges to the rolling bearing in contact with the pulley as disclosed in Japanese Patent Unexamined Publication No. Hei. 4-160225, and thus it accelerates an electrolysis reaction of the water contained in the grease to generate hydrogen ions. The hydrogen ions are attracted to the raceway surface, and enter the inside of the raceway surface in the form of hydrogen ions. This will bring about early exfoliation of the race ring.
Where the pulley entirely formed of resin is used, the weight reduction is advantageously secured. The resin per se is usually an electrically insulating material. Accordingly, it is easy to be charged through its friction against the belt. Foreign materials, e.g., dust, are easy to attach to a belt sliding contact surface of the pulley which is in sliding contact with the belt. The belt sliding contact surface is likely to be worn abnormally through the abrasive action there. Temperature excessively rises on the belt sliding contact surface, so that the surface is abnormally deformed. As a result, the belt power transmission is abnormal or the lifetime of the pulley per is reduced.
The technique disclosed in Japanese Patent Unexamined Publication No. Hei. 3-189450 is such that at least the surface of the pulley which is mounted on a rotatable body is hard chrome plated, and the V-groove portion of the pulley, viz., the belt sliding contact surface, is subjected to electrodeposition coating. In this type of the pulley, part of static electricity generated will flows into the inside of the bearing via the plated part. As a result, discharge is performed between the rolling element and the raceway surface. In this case, if a potential difference between the rolling element and the race ring (inner ring or outer ring) is in excessive of 1V, hydrogen will be generated as in the above case.
The present invention has been directed to solve the problems stated above. It is an object to provide a rolling bearing unit which suppresses an amount of static electricity generated through the friction between the pulley and the belt, and puts a potential difference produced in the rolling bearing to be 1 V or lower by forming a strong insulating layer over a range from the pulley to the bearing, whereby generation of hydrogen ions is suppressed and the lifetime of the unit is elongated.
The above-mentioned object can be achieved by a rolling bearing unit, according to the present invention, having a metallic pulley for slidably supporting a belt on an outer surface thereof, and a rolling bearing for rotatably supporting the pulley, wherein an insulative resin coating film with a thickness of the range of 10 to 40 xcexcm is formed on the outer surface and/or an inner surface of the pulley, and an electrical resistance value of a path ranging from the belt sliding contact surface of the pulley to the inner surface or the outer surface of the rolling bearing is 1 Mxcexa9 or larger. The insulative resin coating film is formed by electrodeposition coating with an insulative resin.
The term xe2x80x9celectrodeposition coatingxe2x80x9d is a called cation electrodeposition coating. In the coating process, a member to be coated is immersed in a water paint, and then the member immersed is set as a cathode, while the paint is as an anode. A DC voltage is applied to between the member and the paint, whereby a coating film is formed over the member. In the coating process, paint enters narrow gaps of the coated member when the coating process specifications are controlled so. A coating film of the range of 10 to 40 xcexcm thick is uniformly formed over the all surface. When the coated member is pulled up from the coating solution, the coating film formed thereon is unsoluble in water and has a low water content.